Defects in magnetic media such as data disks in disk drives can cause data loss or drive damage. Typically, a defect includes a reduction in the magnetic material at a certain point on the surface of a magnetic medium. The reduction of the magnetic material below a standard level results in a reduction of the signal amplitude and is referred to as a "dropout". The defect can also include an increase in the magnetic material at a certain point on the surface of a magnetic medium as a raised feature. The increase in the magnetic material above a standard level results in an increase of the signal amplitude and is referred to as a "dropin". In a disk drive, if the raised feature on the data disk medium is high enough to hit the read heads, the resulting friction causes heating of the heads, which in magneto resistive heads generates an unwanted voltage transient at the output of the head. The transient voltage is referred to as a thermal asperity (TA), and the corresponding defect is referred to as a TA defect. Therefore, it is necessary to accurately identify defects such as dropins, dropouts and TAs during disk drive self-scans so that disk sectors containing defects are mapped out and not used to store data thereon. A case where a defect is not identified is referred to as a "missed defect", and a case where a defect is falsely identified is referred to as a "false alarm". If a sector containing a defect is not mapped out, attempts to read data from the sector can generate a multitude of errors which many error correction codes (ECC) cannot correct. On the other hand, if too many sectors that do not contain defects are mapped out, the capacity of the disk drive is needlessly reduced.
Conventional methods of identifying defects rely on detecting bit errors as indication of defects. A data pattern is written to the disk, and then read back. The data read back is compared with the data pattern for mismatches. Multiple passes are used in order to obtain acceptable defect mapping accuracy. A certain window length is selected such that errors occurring within the same window on different passes indicate a defect in that window. However, a major disadvantage of such methods is their high rate of false alarm. Random noise in reading data from the data disk causes a high rate of bit mismatches, and as such, many sectors are falsely mapped out due to the detection method mistaking noise for detects. The problem is worse in disk drives which utilize lower signal-to-noise ratio, causing significantly higher false bit errors. Further, generally the window must be large enough to account for the worst case spindle-speed variation between subsequent passes. This increases the probability of false bit errors due to random noise occurring within the same window on different passes, and many sectors without defects are unnecessarily mapped out.
Another disadvantage of conventional methods is the length of time required for defect mapping. A shorter defect mapping period reduces manufacturing costs. With any defect identification method, typically it may be necessary to perform more than one pass in order to achieve acceptable missed defect and false alarm probabilities for a desired defect mapping accuracy. A pass would normally include a single write followed by a single read. Other times, only one write is performed with multiple reads, wherein a pass refers to a single read. Repetitive passes allow distinguishing between repetitive errors due to defects and random errors due to noise. As such, greater defect mapping accuracy is achieved when more passes are used. However, improving the defect identification accuracy reduces the number of passes required to achieve the same or better missed defect and false alarm probabilities.
Yet another disadvantage of conventional detection methods is their inability to provide information about the type of defect. In the bit error detection method described above, the detection method only provides information about the location of the error. No information about the type or severity of the error is provided.
There is, therefore, a need for a method of accurately detecting defects in magnetic media. There is also a need for such a method to reduce both missed defect and false alarm probabilities. There is also a need for such a method to reduce the number of required passes. There also a need for such a method to reduce the time required for defect mapping. There is also a need for such a method provide information about the type of defect in a magnetic medium.